A general inflation film manufacturing apparatus forms a synthetic resin film by cooling and solidifying a molten synthetic resin tube extruded from a die slit (for example, see Patent Document 1). The inflation film manufacturing apparatus of Patent Document 1 includes a die head having a ring nozzle, an outer blowing nozzle and a measurement device for measuring the film thickness of tube-shaped film. The volume of air flow passing through the nozzles of the ring of the air nozzle group is controlled based on the measurement results of the measurement device.
Moreover, an inflation film manufacturing apparatus which controls the thickness variation of a synthetic resin film by creating a difference in the volume of cooling air locally in the circumferential direction is proposed (for example, see Non-Patent Document 1). An air ring illustrated in FIGS. 9 to 11, for example, is known as an example of an air ring used in this type of inflation film manufacturing apparatus. As illustrated in FIG. 9, a number of block-shaped slope members are arranged in a ring form in a cooling air passage inside of an air ring 100. The slope members 101 are moved in a radial direction by a moving portion 107 such as a motor, to locally change the gap of passages 103 formed between the slope members 101 and a blow out ring 102. In this way, it is possible to locally control the volume of cooling air directed to a molten synthetic resin tube 104 in a circumferential direction. In FIGS. 9 and 10, reference numeral 105 indicates a hose opening, through which cooling air 106 from a blower (not illustrated) is introduced via a hose (not illustrated) connected to the hose opening 105. In the illustrated example, the hose opening 105 has four hose openings separated at a predetermined interval in the circumferential direction.